The difference in phase between the voltage supplied to an induction motor and the resulting current through the motor is indicative of the load on the motor. It is known for a power control system to be connected to a motor in order to detect and compare these supplied-voltage and resulting-current signals. Based upon this comparison, the power control system may then control the voltage applied to the motor, which in turn controls the flow of current to the motor, in order to reduce the power consumed by a less than fully loaded motor.
U.S. Pat. No. 4,266,177 to Nola, for example, describes a power control circuit for an induction motor wherein a servo loop is used to control the voltage applied to the motor, which in turn controls the flow of current to the motor, in order to reduce the power consumed by the motor. In particular, a pulse signal is used to control the "on" time of a triac which is in circuit with the motor in order to maintain motor operation at a selected power factor. The pulse signal is based upon the measured current-voltage phase angle.
Power factor controllers of the prior art, such as the one just described, use an integrator as part of the processing required to produce the pulse signal. Typically, the integrator includes an operational amplifier and a filter which includes a capacitor and provides a single path of feedback from the output of the operational amplifier to one of the inputs of the operational amplifier. In this arrangement, the value of the capacitor is relatively large, or else the motor may vibrate.
A command signal circuit is also connected to one of the inputs of the operational amplifier, which is typically the same input to which the filter is connected. Conventionally, the command signal circuit contains a potentiometer, which is a relatively expensive electrical component. The potentiometer must be adjusted for the particular motor being controlled in order to provide a proper bias voltage to the operational amplifier. If, as commonly occurs, the potentiometer is incorrectly set, and thus an improper bias voltage is provided, an improper pulse signal will be provided to the triac. This may result in vibration or stalling of the motor, or even failure to start the motor.
Power factor controllers require a power supply in order to provide an operating bias voltage of, for example, 15 volts, to the controller's active components, such as the operational amplifier of the integrator, so that the pulse signal is provided to the triac. In attempting to start the motor, if an improper pulse signal is provided to the triac because the operating bias voltage is not yet up to full potential, the motor may not start, even after a proper pulse signal is later provided.
Power factor controllers of the prior art typically deal with this problem in one of two ways. In particular, some power factor controllers use a transformer as part of the power supply. The transformer provides the full operating bias voltage quickly enough to start the motor. However, a transformer is a rather bulky electrical component that results in oversized control units.
Power factor controllers which do not use a transformer as part of the power supply typically use a resistor and filter capacitor having relatively large values in the power supply. However, because of the large value of the resistor, the power supply generates an excessive amount of heat. Moreover, because of the large value of the filter capacitor, the capacitor takes a relatively long time to charge in order to provide the full operating bias voltage. As a result, the controller may require an additional circuit, commonly referred to as a start-up, or delay, circuit, as described in U.S. Pat. No. 4,266,177. The start-up circuit prevents any power from being applied to the motor until the operating bias voltage is essentially at full operating level.